This invention relates to a method and apparatus for the determination of particle concentration in a particle-bearing sample gas. More specifically, this invention relates to maintaining the rate of flow of the gas relatively constant during sampling so that the volume of gas sampled and therewith the concentration of particles in the gas can be more readily and accurately determined.
In conventional gas sampling devices, such as those used to determine the concentration of solid particulate matter suspended in air, a vacuum is applied to the sample gas so as to generate a flowing stream of gas. A filter of a specific pore size for filtering particles greater than a predetermined size collects particles from the flowing stream of gas. The filter is weighed before and after the sampling period so as to determine the total weight of particles greater than a predetermined size collected during the sampling period. Over a period of time, for example, a 24 hour sampling period, the flow rate past the filter decreases as particles collect on the filter. This flow rate reduction can be quite significant, for example up to 50% over a 24 hour period, when the particles being collected from the gas are substances, such as soot, which tend to clog the collection filter.
In order to determine the average number of particles per unit volume in the sample gas it is necessary to determine the total volume of gas which has passed through the collection filter. Because of the reduction in flow rate caused by the collection of particles on the filter, the conventional practice has been to compute the average flow rate of the gas from measured initial and final flow rates. Obviously such a computation is accurate only if the decrease in flow rate is linear with time.
A recent development in the area of particle-bearing gas sampling devices is the virtual impactor, a dichotomous gas sampler which divides the gas into two flow paths. The gas in the two paths flows at substantially different flow rates and bears particles of different sizes. With this recent development in which a single vacuum source applies a vacuum to the sample gas and the flow of the gas is divided into two paths, each path having its own filter for the collection of particles of a predetermined size, the accuracy of the conventional computational method to arrive at the total volume of gas sampled and thus the particle concentration of the different sized particles, becomes even more suspect.
It is apparent that the computation of particle concentration in a sample gas can be more readily and accurately determined if the flow rate of the stream of gas generated by the vacuum source can be maintained constant throughout the sampling period. One method of maintaining the flow rate of the stream of sample gas constant involves varying the speed of the vacuum source, i.e., the vacuum pump, in response to the mass flow rate sensed proximate the collection filter by a flow rate transducer. As particles collect on the filter and the flow rate decreases, the pump speed must increase to increase the flow rate. Such a method is complex and has proven to have a high failure rate, with a resultant increase in manpower and servicing costs.
In an alternative method, generally used in only high volume gas samplers, a flow regulator sensitive to the dynamic pressure of the relatively high velocity gas, maintains the flow rate constant. The flow regulator comprises a movable circular disk located in the stream between the filter and the pump and having its working surface oriented normal to the direction of the flowing gas. The disk is restrained by a spring which is compressed by the dynamic pressure of the flowing gas acting on the working surface of the disk. The disk is thus movable longitudinally within the conduit confining the stream. The portion of the conduit surrounding the disk has a generally conical shape which converges in a downstream direction so that the area of the annular opening defined by the outer periphery of the disk and the inner wall of the conical portion of the conduit varies with the longitudinal position of the disk in the conduit. Thus as particles collect on the filter and the flow rate decreases, the corresponding decrease in dynamic pressure allows the compressed spring to move the disk away from the converging portion of the conduit, thereby increasing the annular opening and the flow rate.
This alternative method of flow rate control is dependent upon the dynamic pressure of the flowing gas and is thus limited to high volume gas sampling. Additionally, because the dynamic pressure opposing the compressive spring varies with the square of the velocity of the gas and because the spring force is linear only over a relatively small compression, the flow rate is maintained constant only over a small range of flow rates, thereby preventing constant flow rate control during long sampling periods in which substantial flow rate reduction is typical.
While the present invention is generally directed to a method and apparatus for sampling a static body of gas, U.S. Pat. Nos. 2,982,131; 3,859,842; and 3,965,747 disclose methods for sampling a moving body of gas, such as exhaust gases in a flue, in which the sample gas flows through the sampling device at generally the same velocity that it is flowing in the flue, thus permitting the sampling to be conducted isokinetically.